Project Summary Bone metastasis remains a major cause of death among patients with prostate cancer. Unfortunately, current treatments for bone metastases are mainly palliative. A major complication of bone metastasis is bone pain. Although several lines of study have suggested that nerves have a role in cancer progression, and that bone pain and overall survival are negatively correlated, the mechanisms involved remain elusive. We have found that: 1) cancer metastasis to bone or metastatic bone disease enriches sensory nerves that express the neuropeptide calcitonin gene-related peptide (CGRP) in the bone, and that causes bone pain; 2) bone-metastatic cancer cells express elevated levels of calcitonin-receptor like receptor (CRLR); 3) CGRP induces cancer proliferation through the CRLR/p38 pathway; 4) activated FMS-like tyrosine kinase 3 receptor (Flt3) is present in the dorsal root ganglia of mice presenting bone pain indicators; 5) bone-metastatic cancer cells express elevated levels of the Flt3 ligand (FL); and 6) FL induces the sprouting of sensory nerves. We therefore hypothesize that (a) FL derived from bone-metastatic prostate cancer stimulates sensory nerves through Flt3, resulting in cancer-induced bone pain; and (b) CGRP expressed by cancer-associated sensory nerves induces progression of metastatic bone disease through the CRLR/p38 pathway. In this R01 proposal, submitted in response to PAR-16-245, we will: (1) Determine whether bone-metastatic cancer cells increase sensory nerve sprouting and CGRP synthesis in sensory nerves, contributing to cancer-induced bone pain, through the FL/Flt3 axis; and (2) Determine whether CGRP expressed by sensory nerves in bone-metastatic lesions stimulate bone metastatic outgrowth through CRLR/p38. Using an in vitro primary dorsal root ganglia culture system and a unique mouse model of cancer-induced bone pain, will allow us to measure within the same animal: (i) tumor growth, (ii) skeletal innervation, (iii) bone remodeling, and (iv) resultant pain behaviors. Using bone biopsies from patients, we will probe the molecular mechanisms whereby the crosstalk between bone metastatic cancer and sensory nerves controls both progression of bone metastases and development of associated pain. We will use these results to develop a new therapeutic strategy targeting cancer/nerve interactions. In the short term, this study will elucidate new mechanisms of bone metastasis and cancer-induced bone pain. In the long run, a better understanding of how metastatic progression and pain signals influence one another to worsen disease progression will aid in discovering new therapeutic targets for both cancer-induced bone pain and bone metastatic cancer ? areas in which current therapies are wanting ? to decrease suffering and improve the survival of cancer patients with bone metastases.